[R-package] quote path variables in build-cran-package.sh

[jameslamb]

LightGBM provides a script, build-cran-package.sh, which can be used to produce a CRAN-style source distribution of the R package.

This is effectively user-facing code, since we encourage users to run it while developing or building from master to get unreleased features (e.g. #4259 (comment), #4464 (comment)).

I realized yesterday, working on #3946 , that that script can break if called from a path that has spaces in it.

Consider the following:

mkdir -p './some files'
cd 'some files'
git clone --recursive https://github.com/microsoft/LightGBM.git
cd LightGBM
sh build-cran-package.sh

This errors out like so:

build-cran-package.sh: line 15: test: /Users/James.Lamb/repos/open-source/some: binary operator expected
cp: /Users/James.Lamb/repos/open-source/some is a directory (not copied).

This PR proposes quoting all shell variables in that script which include paths, which prevents this problem.

[StrikerRUS]

Thanks! I searched for $ in that file and can confirm there are no unquoted path variables anymore.

[github-actions]

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